1. Field of the invention
This invention relates to barometric altimeters, and particularly to those accurate barometric altimeters based on the electronic technology to be used for aircrafts.
2. Description of Prior Art
The principle of barometric altimeters is based on the fact that the atmospheric pressure decreases as the altitude increases, and the altitude is obtained by measuring the atmospheric pressure.
In most of the conventional barometric altimeters, the atmospheric pressure is measured by the deformation of an aneroid capsule due to the change of pressure, and this deformation is indicated after being mechanically magnified by a train of gears. As the deformation is magnified greatly, even slight friction in the mechanism causes an error. Because this error is not proportional to the altitude and does not decrease even at low altitude, it may cause a danger in low altitude flight of an aircraft.
In order that the indicator be graduated in a linear scale, a logarithmic transformation mechanism also providing the mechanical magnification, is necessary and the accuracy needed for this mechanism is very critical.
In those applications where the digital value of altitude is required, such as automatic altitude reporting system, a complex high cost encoder, such as optical type one, has been used.
The applicant has made an invention as is disclosed in the U.S. Pat. No. 3,729,999 in which no mechanical magnification nor logarithmic trans-formation is needed and accurate indication of altitude is obtainable by means of electronic procedures according to the International Civil Aviation Organization's (ICAO) standard atmosphere. Although the above invention is useful where a pressure sensor produces an output signal linearly proportional to the atmospheric pressure of the altitude to be measured, some types of accurate and sensitive pressure sensors do not produce output signals which are directly, or linearly proportional to the atmospheric pressure sensed. Also the above invention is based on a counter oriented philosophy and results in some complexity in the circuitry used.